Nonlinear seismic response in the western L'Aquila basin (Italy): Numerical FEM simulations vs. ground motion records

•2D subsurface model of the western L'Aquila basin (Italy) is proposed.•Analysis of the near-fault weak- and strong-motion records is executed.•2D FEM simulations are performed.•Observed and computed results are compared to assess the seismic response.

In the western L'Aquila basin (Italy), six seismic stations of the Italian Strong Motion Network, which were used on both soil and rock outcrops, recorded a huge amount of weak- and strong-motion data near an active fault during the 2009 L'Aquila seismic sequence. This paper analyzes the ground motion records of events with magnitudes ranging between 1.6 and 6.3 and compares the observed peak accelerations and the H/V and V/H spectral ratios with those revealed from numerical simulations. The finite element method is considered herein to perform 2D dynamic modeling on a geologic cross-section of the upper Aterno River Valley, which is 1000 m wide and 50 m deep, based on the reviewed geologic, geotechnical and geophysical investigations that were performed in previous studies. The Martin–Finn–Seed's pore-water pressure model is used in the simulations. The observed peak accelerations, the frequencies and the levels of maximum amplification are correctly estimated using the FEM simulations. The consistency between the computed and the experimental results shows a seismic response that is sensitive to the level of earthquake magnitude and mainly depends on the rock and soil elastic and dynamic properties, the intensity and the depth of the seismic impedance contrast, the water table depth, the two-dimensional irregular configuration, the dip angle of strata, the shape and the frequency content of the input motion record. The seismic motion amplification is usually higher at station AQV, which is located near the valley center, where a peak acceleration value of 0.66 g was recorded. Both the PGA amplification factor and the maximum amplification frequency of the horizontal component decrease when the earthquake magnitude level increases, which shows a nonlinear seismic response at the stations AQG (on fractured and weathered rock), AQA and AQV (on alluvium). The simulation of the 2009 Mw = 6.3 L'Aquila earthquake confirms the observed nonlinear effective stress behavior and allows us to relate it to the generation of excess pore-water pressure, which ranges between 50 and 300 kPa.